This invention relates to apparatus for generating high voltages and, more particularly, to a high frequency high voltage power supply with means for controlling or varying its output power.
High voltage generators or power supplies are required for various types of applications, for example, to energize a magnetron in a microwave oven. One such power supply apparatus is described in U.S. Pat. No. 4,628,284, which issued Dec. 9, 1986 in the name of Bruning and Fellows, the disclosure of which is hereby incorporated by reference. As will become apparent from the description below, the present invention is in some respects an improvement over the high frequency high voltage power supply described in that U.S. patent. The patent discloses a current-fed parallel resonant oscillator that includes a pair of push-pull operating transistors coupled to a parallel resonant circuit that includes a transformer having a center-tapped primary winding and a capacitor connected in parallel therewith. The transformer primary voltage is given by the following expression, V.sub.pri =V.sub.cc .pi.T/2ton where V.sub.pri =peak primary voltage; T=switching interval; t.sub.on =on-time of one transistor. The voltage envelope over a long time period (&gt;&gt;T) is of constant amplitude.
The power delivered to the load is then ##EQU1## where: ##EQU2## assuming ideal coupling;
and where R.sub.L is the load resistance connected to the output terminals of the power supply. The output power can therefore be controlled by controlling the transformer primary voltage.
Consumer microwave ovens generally employ a magnetron energized by a ferroresonant power supply operating at the power line frequency to supply microwave heating energy to the cooking cavity of the oven. The output power of this power supply is discontinuously controlled by means of a control circuit which disconnects the 60 Hz AC supply voltage in order to vary the average microwave heating power applied to a load in the oven cavity. In one mode of operation, the magnetron will typically be pulsed on for approximately one second and will be pulsed off for approximately 10 seconds. As a result, the magnetron heater filament is repetitively heated and cooled which produces stresses on the magnetron which reduce its useful life. It would therefore be advantageous to provide a means for varying the average microwave power applied to a cooking load in a smooth and continuous manner so as to keep the magnetron heater filament energized at all times during operation of the oven. The high voltage power supply described in the patent solves the problem by providing continuous power control using a self-sustaining power oscillator with inherent self regulation of its output power.
Another apparatus for generating high voltages is disclosed in U.S. Pat. No. 3,417,314 which issued on Dec. 17, 1968. This patent describes a high-efficiency, high voltage generator consisting of a DC voltage converter that includes a transistor oscillator that is gated on and off by a gating circuit so that the oscillations are interrupted periodically. As a result, the rms tank voltage is zero for a part of each period of operation of the apparatus.